Hidden Chill - Why your Bay Window is a Cold Spot
The persistent problem - Cold & Damp Bay Windows in Solid Wall Homes
Gain a 50% thermal uplift
Older homes in Surrey often suffer from heat loss and black mould, with solid walls and bay windows being some of the main culprits.
At Skim 2 Fin Plastering & Wall Insulation, we’re experts in insulating bay windows across Cobham & Surrey. We offer tailored solutions designed to tackle draughts and improve energy efficiency.
Many people don’t realise that uninsulated bay windows can be responsible for up to 50% of heat loss in older houses. This not only drives up energy bills but also creates the perfect conditions for damp and mould, making your home less comfortable and healthy.
Timber Frame & Tile Bays
Oriel-style bay windows, which project from the main wall of a building, often lack proper insulation between the timber frame and outer tiles. They also typically have no built-in draught seals, making them particularly cold and prone to issues like black spot mould. This mould commonly forms due to condensation building up on the cold interior surfaces.
Another downside to this design is the poor sound insulation. With little to no mass or insulating materials, traffic noise and other airborne sounds can easily pass through into the home.
Rooms with this type of bay are usually freezing in the winter and difficult to keep warm. As shown in the case study video below, many homeowners try to tackle the problem themselves using draught-proofing products like sealants, weather strips, or even fleece-lined wallpaper. Unfortunately, these DIY solutions don't significantly improve the thermal performance of the bay, so cold spots and condensation issues continue to be a problem.
Solid Brick & Render Bays
Older properties with solid walls and damaged or cracked render are especially vulnerable to cold, damp, and mould issues. Without proper insulation, solid brick walls and bay windows offer very little resistance to heat loss, making these homes harder to keep warm and less energy-efficient. Like timber and tile bay constructions, the cold interior surfaces in these areas often cause condensation in colder months, creating ideal conditions for damp and black mould to thrive.
Cracked render makes things worse by allowing rainwater to seep into the walls. This moisture can become trapped, making the damp problem more severe. In more serious cases, it can even weaken the structure of the bay over time.
The best way to tackle these problems is by repairing any damaged render, installing internal wall insulation, and improving ventilation to help control moisture levels. These upgrades not only create a more comfortable living space but also lead to noticeable savings on heating bills.
Are your solid walls cold and attract condensation?
Damp & Mould
Black mould forming around old bay windows is usually the result of a mix of moisture, poor airflow, and a lack of proper insulation. Here’s a clearer look at what typically causes the problem:
1. Condensation
Older bay windows often have little to no insulation, making their surfaces much colder than the rest of the room. When warm, moist air hits these cold surfaces, it condenses into water droplets—creating the perfect breeding ground for mould.
2. Poor Ventilation
When airflow is limited—due to heavy curtains, furniture positioned too close, or blocked vents—moisture gets trapped in the bay area. This trapped dampness encourages mould growth.
3. Penetrating Damp
Water can enter through gaps in window frames, cracked seals, crumbling mortar, or damaged render. High ground levels and poorly maintained window joinery, especially timber, can soak up water and make things worse.
4. Lack of Insulation
Many older bay windows are poorly insulated or not insulated at all, creating cold spots that attract condensation and mould. Thermal bridging—where heat escapes through materials like metal or timber—further adds to the problem.
5. External Defects
Blocked gutters, leaky downpipes, and cracks in exterior walls can allow water to seep into the structure. This increases the risk of damp forming around and inside the bay window.
6. High Indoor Humidity
Everyday activities like cooking, showering, or drying clothes indoors add moisture to the air. Without proper ventilation or extraction, this moisture settles on the coldest surfaces—often the bay windows—triggering condensation and, eventually, mould.
Tackling the root causes—by improving insulation, repairing defects, and ensuring good ventilation—can significantly reduce mould growth and improve overall comfort in the home.
